1. Field of the Invention
This invention relates to a roller fixing device, and more particularly a heat roller fixing device having an improved claw member.
2. Description of the Prior Art
Generally, in an image formation process using a recording device, for instance, an electrophotographic copying machine, a visible image is formed by forming an electrostatic latent image corresponding to a document image with a corona charge and a light irradiation on a charge receptor using an insulating photoconductive material or the like as a photosensitive member, and developing it with a toner developer containing colored fine thermoplastic powders as a main ingredient. The visible image or toner image thus formed on a charge receptor is transferred to a transfer material such as a paper.
The toner image transferred on the transfer material as described above, is fixed on the transfer material with the conventional heating means and the image formation process is completed.
As a heat means in the heat fixing device as described above, a heat roller fixing device is widely used from the reasons that there exists no danger of firing since said device is comparatively simple in mechanism, high in heat transfer efficiency and is possible to lower the set temperature.
The heat roller fixing device having such advantages generally consists of a heat roller having a heater therein and a pressure roller in contact with said heat roller under pressure, and the surface of said heat roller in contact with the toner is coated with a heat resistant resin such as Teflon (Trade Mark) or the like in order to improve releasability from the toner and impart heat resistance thereto, while the pressure roller which contacts with this under pressure must have a certain nip width with said heat roller as well as releasability and heat resistance. Accordingly, the pressure roller is required to be resilient and, for instance, the lining with a silicone rubber is generally performed.
However, even when the heat roller having an improved releasability is used, the transfer material may wrap around the roller, because small amounts of toner adhere and pile on the surface of the roller by fixing a copy on which the toner is given extensively or by repeating the fixing for a long period and eventually both rollers are stained.
As a means to prevent the wrapping phenomenon, providing a plurality of separation claws adjacent to the roller is believed to be an effective method.
On the other hand, in accordance with the recent tendency toward a high-speed copying machine, attempts have been made at shortening of copying time per one sheet by carrying transfer material in the direction along its shorter side to increase the amount of the transfer material to be processed within a given time.
Consequently, the roller fixing device of the copying machine has had to be lengthened in accordance with a longer side of the transfer material, and a new problem on its mechanism such as bending of the roller has occurred. Furthermore, when, for example, thin transfer material is used, curling of the paper, particularly wave curling also becomes remarkable.
FIG. 1 is an exterior view which shows a constitution of the conventional heat roller fixing device.
In FIG. 1, numeral 1 is a driven heat roller, and a coating layer 2 made of Teflon or the like is provided on the surface thereof. Numeral 3 is a heater housed within the roller 1. Numeral 4 is a pressure roller which can contact with said heat roller 1 under pressure and move therewith, and on the surface of said pressure roller is provided a lining layer 5 consisting of silicone rubber 5 mm thick. Said heat roller 1 and pressure roller 4 contact under pressure with each other only during fixing operation by a pressure operating device (not shown), or affords a nip width N while being always contacted under pressure. Numeral 6 is an upper separation claw which is provided so that the top end portion thereof may contact under pressure with the surface of the heat roller 1 to prevent the transfer material 8 from wrapping around the heat roller 1. Numeral 7 is a lower separation claw which is provided so that the top end portion thereof may contact with the pressure roller 4 with the same object as that of the upper separation claw 6. The material for said separation claws 6 and 7 is preferably heat-resistant resin, and the surface of the top end portion thereof may be coated with a fluorine-containing resin in order to prevent the toner from adhering thereon.
The separation claws 6 and 7 are pivotably supported by guide plates 9 and 10, respectively. The angle formed between the top end portion thereof and the roller is designed so that the top end portion of said claw may not damage the surface of the roller and may separate effectively considering the nature of material for counterpart rollers 1 and 4.
When thin transfer material 8 is used in the heat roller fixing device constituted as described above, wave curling occurs, after it has passed through the nip width of the roller, in a region with a length L between the endpoint of said nip width and the point where the top end portion of the lower side separation claw 7 is brought into contact with the surface of the pressure roller 4.
One cause of the occurrence of the wave curling is overheating by the fixing device. In other words, in recent copying machines, the basis weight of the transfer material used is progressively increasing and accordingly the machine is designed to be capable of handling thick transfer material such as one with 110 kg basis weight. As a result, more heat energy than heretofore is given to the fixing device to accomplish a sufficient fixing even when the thick paper as described above is used.
Therefore, when thin transfer material of, for instance, 45 kg basis weight is used, excessive heat is inevitably given in fixing, resulting in the lowering of stiffness of the transfer material and occurrence of the curling.
When such transfer material passes through the nip width of the roller as described above, wave curling occurs by the fluctuation of the transfer material between the endpoint of said nip width and the point where the top end portion of the lower separation claw is brought into contact with the pressure roller surface (L in FIG. 1).
In the conventional fixing device, said distance L is approximately 10 mm.